Simple Contact Mike

There are plenty of online guides for making contact microphones from piezo discs to listen to the microscopic vibrations of objects. (Steve Schaberg’s guide is great for first-time builders.)

My version includes a limiter to protect sensitive mic inputs from high-voltage spikes. There are many better designs, including fully buffered FET preamps, but if you’re looking for a “Radio Shack” solution hat won’t fry sensitive mic inputs, this will do!

Radio Shack Parts list

Piezo Element (part # 273-073)
(Consider buying 2 because it’s easily broken during construction. There are many types of piezo buzzer and only some will work. Avoid anything thicker than a lollipop because it contains extra electronics that we will throw away.)

6ft Mono 1/8″ to Mono 1/8″ extension cable (part # 42-2472)
(Longer is OK, stereo is OK, but it must be 1/8″ and have a male end and a female end)

XLR Audio Connector (male) (part # 274-010)
(This will allow you to plug your contact mic into professional recorders. If you own a recorder with 1/8″ inputs, don’t bother with this part.)

Remove Piezo Disc From Plastic Case

It’s much easier to buy raw discs, but Radio Shack doesn’t sell them.

Flip the piezo element so the back is facing up.

Use a sharp knife to score around the rear seam.

Use a jeweler’s screwdriver to gently pry off the back. Be careful not to bend the disc inside!

Once the disc is free, you can discard the plastic.

Prepare L.E.D.s

LED’s are usually used to generate light, but in this case we’re using them to limit the voltage that the mic produces. Otherwise, in rare cases, a hard strike on the piezo disc could send a damaging voltage spike into the recorder.

Note that the L.E.D.s have a long leg and a short leg.

Arrange a pair so their short/long legs are mismatched (short touches long, long touches short).

Twist the pairs together and solder each leg so it doesn’t fall apart.

Prepare Cable

The 1/8″ mono extension cable should have a male end and a female end.

Cut the cable about 5″ from the female end.
(This will become the 1/8″ to XLR adapter.)

Note the internal structure of the cable. The braided shield is the “ground” and the center conductor is the “hot” or “signal” wire.

Strip the insulation on both of the cut ends of the cable. See later diagrams to get a feel for length.

Solder 1/8″ to XLR Adapter

Dis-assemble the XLR plug.
(You don’t need to take off the black rubber part.)

Thread the 5″ cable through the XLR shell.

Solder the center conductor to XLR pin 2, and the shield to XLR pin 1.(Theoretically you should also connect the shield to XLR pin 3 but most recorders don’t mind.)

Re-assemble the XLR plug and tighten the screws near the black rubber part.

Solder Contact Mic

Lay out the parts as shown in the diagram.(The red wire from the piezo disc must attach to the cable’s center conductor, not the shield. The L.E.D.s can be flipped either direction because they have no polarity.)

Wrap the wires around the L.E.D. leads, then solder them in place.
(Attach the wires close to the L.E.D.s as shown in the diagram. Long leads will eventually get mangled and short out!)

Snip off the excess L.E.D. leads that are sticking out on either side.

Plug in the mic and test it. Before you plug it in, make sure the recorder’s 48v phantom power is off!

After testing, wrap the connections in electrical tape to prevent short-circuits. There are more permanent ways to do this, but it’s smart to make things easily accessible since the discs are fragile!